1. Field of the Invention
The present invention relates to an automatic predistortion adjusting circuit.
2. Description of the Related Art
A predistortion circuit is provided to compensate for a non-linear spectrum generated in a high frequency amplifier. In this case, the non-linear characteristics of the predistortion circuit are opposite to those of the amplifier. Generally, the non-linear characteristics of the predistortion circuit are automatically adjusted during the assembly process of the amplifier, and thereafter, the non-linear characteristics of the predistortion circuit are not changed during operation.
A prior art automatic predistortion adjusting circuit is constructed by a predistortion circuit and a main amplifier connected in series. In addition, a directional coupler takes out a part of the output signal of the main amplifier as a branch signal, and a bandpass filter passes the non-linear spectrum of the branch signal. Then, a detector detects the strength of the output of the bandpass filter, and a control circuit controls the non-linear characteristics of the predistortion circuit in accordance with the level of the output signal of the detector, so that this level is minimal, i.e., brought close to zero. This will be explained later in detail.
In the above-described prior art, however, in the case of multi-carriers, although the bandpass filter can pass the non-linear spectrum therethrough wile completely attenuating the desirable spectrum, the passband frequency (stop-band frequency) of the bandpass filter depends upon the frequency of the input signal, so that the characteristics of the circuit fluctuate in accordance with the input signal.
On the other hand, in the case of a multi-quadrature amplitude modulation (QAM) signal included in an input signal, a non-linear spectrum such as a third inter-modulation distortion spectrum have a larger bandwidth than that of a desirable spectrum. Therefore, since the frequencies of the non-linear spectrum are superposed onto those of the desirable spectrum, the non-linear spectrum cannot be removed by the bandpass filter. As a result, it is difficult to control the predistortion circuit.
Further, it is difficult to manufacture a bandpass filter having ideal bandpass characteristics suitable for the automatic predistortion adjusting circuit.